A controllable pitch ship propeller is designed such that the angle of attack of the blade can be continuously varied. In this manner, the torque of the main engine may be varied. A controllable pitch propeller is common for medium sized ships (50-150 m l.b.p.) with medium to high requirements on maneuverability.
A controllable pitch propeller is often combined with a shaft generator connected to the main engine via a gear box. When operating such a configuration, the propulsion effect is adjusted solely by varying the pitch of the propeller blades. The engine speed of the main engine is kept constant in order to maintain the generator frequency within allowable limits.
As long as the ship is operating close to its design speed, a fixed engine speed is not a problem from an efficiency point of view, but at lower speed of the ship a full engine speed and a low torque provides a substantially lower efficiency of the propulsion system as a whole. At lower speed of the ship, so called “slow steaming”, it is from an efficiency point of view appropriate to vary both the pitch and the engine speed, a so called combination operation.
The majority of present operating systems for controllable pitch propellers have a combination condition wherein both the pitch and the engine speed can be controlled simultaneously using the same operating lever. The relationship between the pitch and the engine speed is fixed and is calculated with a margin for different load conditions and in order not to exceed the load limit curve of the engine. During operation in a combination condition, the shaft generator cannot be used, but electricity can instead be generated using any one of the ship's auxiliary engines.
The fixed combination curve has the disadvantage that it is calculated with a margin to the maximum allowable load for the engine. This results in that the maximum efficiency of the engine only can be achieved under one condition at the most.
Moreover, most of the existing control systems have a safety function, a “Load Control”, limiting the maximum torque for the main engine in order to limit the pitch from exceeding a value that can be set. This renders the engine speed higher and the torque lower than what is optimal. See JPS598590 CONTROLLER FOR MARINE ENGINE.